Technical Field
A process for preparing a clustered functional polyorganosiloxane comprises reacting an aliphatically unsaturated species, a species containing silicon bonded hydrogen atoms, and a reactive species in the presence of a hydrosilylation catalyst. The clustered functional polyorganosiloxane prepared by this process has improved physical properties (e.g., increased tensile strength and % elongation) as compared to a ‘dumb-bell’ type polyorganosiloxane prepared by a different process. The clustered functional polyorganosiloxane can contain a filler and still exhibit improved dispensing properties as well as the improved physical properties over a ‘dumb-bell’ type polyorganosiloxane prepared by a different process.
Background of the Invention
Polyorganosiloxane compositions that cure to elastomeric materials are well known. Such compositions may be prepared by mixing polydiorganosiloxanes having curable (e.g., hydrolyzable, radiation curable, or heat curable) groups with crosslinking agents and/or catalysts, as needed. Generally, the polydiorganosiloxanes may have 1 to 3 reactive groups per chain end. Compositions including these ingredients can then be cured, for example, by exposure to atmospheric moisture, exposure to radiation, or exposure to heat, depending on the curable groups present.
The cure rate of a particular composition depends on various factors including the type and number reactive group(s) present. It is known that different groups have different reactivities. For example, in the presence of moisture, a silicon-bonded acetoxy group will usually hydrolyze more rapidly than a silicon-bonded alkoxy group when all other conditions are the same. Furthermore, even the same type of curable group can have different reactivities depending on the number of those curable groups bonded to a particular silicon atom. For example, if a polydiorganosiloxane has three silicon-bonded alkoxy groups bonded to one silicon atom on a chain end, then the first alkoxy group is generally most reactive (reacts most quickly), but after the first alkoxy group reacts, it takes a longer time for the second alkoxy group bonded to the same silicon atom to react, and even longer for the third. Therefore, there is a continuing need to prepare clustered functional polyorganosiloxanes having more of the “most” reactive groups per molecular terminus.
Furthermore, to show utility for certain applications, such as silicone adhesive applications, a filler may be added to the composition to improve the physical property profile (e.g., increase tensile strength and increase % elongation to break) of the resulting cured product of the composition. The nature of the filler, its chemistry, particle size and surface chemistry have all been shown to influence the magnitude of the interaction between polyorganosiloxanes and the filler and consequently the ultimate physical properties. Other properties such as adhesion and dispensability also play a role in the performance and commercial acceptance of a composition for adhesive applications. Silicone adhesives generally have tensile properties in excess of 200 pounds per square inch (psi) and 100% elongation, with adhesion to a wide variety of metal, mineral and plastic surfaces.
The synthesis of ‘dumb-bell’ silicone polymers, in which long polymer chains are capped with cyclic, linear and star-shaped species having one or more organo-functional groups has been disclosed. Such polymers have been described which can undergo a number of cure chemistries, e.g., epoxy (glycidyl, alkylepoxy, and cycloaliphatic epoxy), methacrylate, acrylate, urethanes, alkoxy, or addition.
It is desirable to make multifunctional end blocked polymers (clustered functional polyorganosiloxanes) in which the curable groups are clustered at the ends/termini of the polymers. The combination of clustered functional groups with nonfunctional polymer chains separating them in the ‘dumb-bell’ silicone polymers may provide higher physical properties with the minimum drop in cure rate. This approach has been demonstrated for ‘dumb-bell’ silicone polymers in which the curable groups are the same (for example, all curable groups clustered at the polymer chain ends may be either epoxy or alkoxy). This approach has also been demonstrated for so called ‘multiple cure’ systems in which the curable groups differ, for example, all curable groups clustered at the polymer terminals may be a combination of epoxy and alkoxy groups.
In known processes for making these ‘dumb-bell’ silicone polymers, these polymers are prepared in multiple steps. First, a silicone hydride functional ‘dumb-bell’ intermediate is prepared via the reaction of vinyl-end blocked linear polyorganosiloxanes with cyclic, linear or branched silicone hydrides. This initial step is followed by the addition of a reagent to neutralize the platinum group metal catalyst and/or a purification step to remove the silicone hydride functional ‘dumb-bell’ intermediate from the unreacted species and by-products because continued presence of the catalyst at elevated temperatures over time leads to gelation, typically through the ring opening of the cyclic end blocks or crosslinking of the remaining silicon bonded hydrogen atoms. Several approaches to prevent gelation include using platinum group metal catalyst poisons, silylating agents, or catalyst inhibitors such as diallyl maleate, to deactivate the catalyst after the silicone hydride functional ‘dumb-bell’ intermediate is formed. This allows purification (e.g., by stripping or distillation) of the intermediate at elevated temperature to remove solvents, unreacted silicone hydrides, and reaction by-products. The problem with this solution is that it makes it necessary to add more platinum group metal catalyst (an added expense) and higher temperature, typically >80° C. for longer periods to achieve subsequent reactions with unsaturated organo-functional moieties, thereby increasing the time required to perform the process. The increased temperature can be particularly problematic in small ‘dumb-bell’ species in which a large exotherm is typically associated with the hydrosilylation process. The elevated initiation temperature and large exotherms make heat management in industrial process problematic. The unsaturated groups (e.g., methacrylate, acrylate, vinyl or allyl) can autopolymerize via unwanted radical process. These radical process can be mitigated by the addition of polymerization inhibitors such as hydroquinone (HQ), 4-methoxyphenol (MEHQ), butylated hydroxytoluene (BHT), phenothiazine (PTZ), etc. However, in the case of methacrylate functional materials even with radical inhibitors present there is a high incidence of methacrylate autopolymerization in this multiple step process. For these reasons, there is a need for a process with minimal process steps and minimal thermal history, as this may reduce or eliminate the tendency for ring opening and autopolymerization. This may allow greater latitude in organic functional groups, as well as reduce cost due to the additional platinum group metal catalyst and time consuming process steps associated with the known processes.
Furthermore, uses of the ‘dumb-bell’ silicone polymers produced by known the process described above have been limited due to the difficulties associated with formulating fillers with these polymers. The addition of fillers increases the physical properties of cured silicone products such as rubbers, sealants, and adhesives. The fillers, exemplified by fumed silica, used in such compositions are inherently hydrophilic due to surface silanol groups, which lead to severe problems in processing and use. The water affinity of the silanol groups can lead to dielectric breakdown, corrosion, and gelation of adhesion promoters, catalysts, and coupling agents. The silanol groups also lead to larger than required interactions between the polyorganosiloxanes and the silica surface that result in material with too high of a viscosity to dispense or handle, and which exhibits crepe hardening. It is therefore common practice to use fumed silica, which has been pretreated with hydrophobic species to mitigate these problems. These fillers can be pretreated by the silica manufacturers, e.g., Cabosil® TS-530 and TS-720 are fumed silicas treated with hexamethyldisilazane (HMDZ) and polydimethylsiloxane, respectively, from Cabot Specialty Chemicals, Inc. of Billerica, Mass., USA, or the fillers can be treated in situ as part of the production process.
Despite the advantages of fumed silica, product formulators have cited problems with both hydrophobic and hydrophilic forms of fumed silica. For example, because of its moisture-absorbing silanol groups, hydrophilic fumed silica tends to cause problems in electronic adhesives or coating applications because the increased water concentration decreases electrical resistance. Additionally, in coating applications, this moisture, introduced by the hydrophilic fumed silica, can accelerate corrosion of the coated substrate. Also, the shear-thinning efficiency of hydrophilic fumed silica is often inadequate, which is thought to result from adsorption of the liquid onto the fumed silica surface, preventing silica aggregation and thus shear-thinning.
It is possible to make the ‘dumb-bell’ polyorganosiloxanes first and then, in a subsequent process step, disperse a desired amount of filler to get high tensile properties. However, the shear encountered in dispersing the treated filler leads to the formation of new untreated silica, hence silanol groups, as the particles are broken and fractured. This may lead to instability in the rheology and storage properties of the ‘dumb-bell’ polyorganosiloxanes and may also reduce the adhesive properties of a composition containing the ‘dumb-bell’ polyorganosiloxanes, and cured products thereof, to metals. Fumed silicas can also be cumbersome to process during manufacture. When adding fumed silica to a composition containing a ‘dumb-bell’ polyorganosiloxane described above, even high shear blending may be insufficient to disperse the fumed silica particles. It is often necessary to add a process step where the fumed silica-polymer mixture must be passed through a media mill or a three-roll mill to sufficiently disperse the silica. Furthermore, there is the danger of overdispersing fumed silica which breaks the silica aggregates, exposing untreated silica surfaces, and ruining the shear-thinning properties. Eliminating the need for these extra process steps as well as avoiding the danger of overdispersing the silica would be highly desirable to product formulators. Therefore, there is a need in the electronics industry for a process to synthesize a clustered functional polyorganosiloxane in the presence of a treated filler.